Biaxially oriented polypropylene (BOPP) metallized films used for packaging applications often perform multiple functions. For example, in a lamination these films can provide light, moisture, and oxygen barrier for gas-flush applications. In addition, these films can provide a heat sealable inner layer for bag forming and sealing, or a layer that is suitable for receiving an adhesive or inks either by coating or laminating.
It is desirable for the BOPP metallized film to have adequate adhesion between the vapor-deposited metal layer and the film's metal receiving layer. Often, the higher the bond strength between the film's receiving layer and the metal layer, the better. Higher metal adhesion can result in a more robust film in terms of barrier properties and lamination bond strengths. More specifically, by improving the metal adhesion, barrier properties are improved by minimizing the amount of metal pick-off and loss during the vacuum metallizing process and rewinding. Package lamination bond strengths are improved when a metallized film with high metal adhesion is used in a multi-layer lamination, since the interlayer bonds strengths of the laminate are improved. In thick multi-layer bags with many gussets and folds in the sealed areas (e.g. stand-up pouches), the metal layer often delaminates from the BOPP metal receiving layer. This can cause packaging failure or result in a “bag within a bag” phenomenon where the multi-layer laminate structure of the package has one of the layers delaminating. Accordingly, high metal adhesion to BOPP films is highly desirable.
Aesthetic appearance is also important. It is desired that the metallized BOPP film's metal surface has a bright, shiny, reflective appearance, and not a matte, dull, or pewter-like metal appearance. Such a shiny metal appearance is especially desirable when the package includes bright, reflective metal in the finished graphics. A bright, shiny metal appearance can make some print graphics look brighter with more visual “pop”.
Another aspect of metallized BOPP robustness is to ensure that the metal layer does not “craze” during extrusion lamination processes. Due to the high heat load from the molten polyethylene that extrusion lamination can impart to the metallized BOPP film, if the metal receiving layer melts or deforms from this heat load, the metal layer can fracture and crack. This can, in turn, degrade or destroy gas and moisture vapor barrier properties of the film. Onset melting point is the temperature at which a polymer begins to show a phase change. It is desirable to design the metal receiving layer to have a relatively high onset melting point to help avoid issues with thermal shock caused by extrusion lamination.
U.S. Pat. No. 6,190,760 involves low crystalline content propylene homopolymers with a heat of fusion of 20-90 J/g as a metal receiving layer that provides high metal adhesion properties. However, it has been found to suffer from a matte metal appearance and to be prone to crazing during extrusion lamination.
U.S. Pat. No. 5,194,318 describes a HDPE as the metal receiving layer on a polypropylene substrate. Metal adhesion and appearance have been good; however, it has been found that in some high-speed extrusion lamination applications, crazing of the metal can occur, particularly if the HDPE receiving layer is over 3G (0.75 um) in thickness.
U.S. Pat. No. 5,698,317 describes blends of propylene-based polymers with petroleum or hydrocarbon resins as a metal receiving layer. Metal adhesion and appearance are adequate; however, processability is an issue with the low molecular weight petroleum resins causing: die build-up issues, smoke generation, and a tendency to stick to downstream rollers in tentering operations.
U.S. Pat. No. 5,922,471 describes propylene random compolymers of ethylene or butene as a metal receiving layer. However, these materials are prone to metal crazing during extrusion lamination due to their relatively low melting points. In addition, these materials tend to have poor barrier properties. Moreover, the copolymers used are predominantly isotactic whereas the APAO materials are predominantly atactic.
U.S. Pat. No. 5,958,566 describes an ethylene-propylene copolymer with a low ethylene content of 0.05-0.8 wt % as a metal receiving layer. The metal adhesion of such a layer continues to be relatively low being less than 100 g/in.
U.S. Pat. No. 6,033,786 describes blends of propylene-butene copolymers and ethylene-propylene copolymers combined with flame treatment as a metal receiving layer. However, such a blend with its relatively high ethylene and butene content (e.g. over 1 wt %) and low onset melting point can be prone to metal crazing.
This invention seeks to avoid some of the disadvantages of the prior arts and offer an improved metal receiving layer formulation.